1. Field of the Invention
The present invention relates to the registration of color images in a color image output terminal. More particularly, the invention relates to an improved color image alignment system and an improved method and apparatus for detecting registration errors between color separations and a method and apparatus to correct certain color registration errors.
2. Description of the Related Art
Image registration is an important and difficult problem in a xerographic color image output terminal. In FIG. 1, a color image output terminal 10 is shown having four photoreceptors 12, 14, 16 and 18. Each photoreceptor carries a unique color separation obtained by a conventional xerographic processor having charge device 20, write device 22 and develop device 24. The four color separations are transferred to intermediate belt 26 so as to coincide with one another and produce a full color image. Subsequently, the color image is transferred to paper and the color image is fixed thereon. Alternatively, belt 26 can be a copy sheet conveyor so that the four color separations are transferred directly to the delivery medium. Photoreceptors 12, 14, 16 and 18 are driven by rotating members 1, 2, 3 and 4 respectively. Belt 26 is driven by rotating member 5.
In order to deliver good quality images, strict specifications are imposed on the accuracy with which the color image output terminal 10 superimposes the various color separations which compose the individual images. This juxtaposition accuracy is often called registration. In the trade, a limit of 125 micrometers is considered a maximum for acceptable misregistration errors of quality pictorial color images and a 75 micrometer limit is often imposed as a limit by the manufacturers of top quality equipment. These numbers represent the diameter of a circle which would encompass all supposedly homologous color dots.
In a single pass image output terminal, the various color separations are produced by separate imaging members and are passed to the intermediate belt where they are collected in juxtaposition. Registration errors can arise from motion errors of the collecting device and from mismatch of the individual color separations from the imaging device.
With respect to the motion of the collecting device, good registration goals are attainable if the unit is designed such that its kinematic errors are made synchronous with the spacing distance between successive image transfer points of the photoreceptors 12, 14, 16 and 18 and belt 26. In this manner, the modulation of the surface motion is repeatable (synchronous) with the imaging pitch and color-on-color separation errors are minimized. Although the absolute position error of each color may be large, the relative position error between colors is held to specification. The absolute image distortion is usually tolerable. In tandem image output terminals, where the separations are generated and developed on individual photoreceptors and then transferred to an intermediate belt, a mismatch in the motion errors of the photoreceptors contributes to misregistration.
In tandem image output terminals, where the separations are generated and developed on individual photoreceptors and then transferred to an intermediate belt, problems arise due to mismatch in the motion errors of the photoreceptors and due to the photoreceptor eccentricity and wobble. The mismatch contributes to misregistration in the process direction; the eccentricity contributes to variable lateral magnification error; and wobble contributes to lateral registration variations. The eccentricity and wobble contributions exist only in machines where the writing is performed by a light beam scanning through a finite angle (usually called ROS for Raster Output Scanner). Image bars do not present these problems.
One common way of improving registration is described in U.S. Pat. No. 4,903,067 to Murayama et al. Murayama et al. employ a marking system with a detector for measuring alignment errors and mechanically move individual color printers to correct misalignment.
Color printers that employ marks produced by each of the constituent colors in juxtaposition with each other enable correction of lateral and longitudinal relative position, skew and magnification. The marks may be machine readable, and data may be processed to measure registration errors for the purpose of automating registration error correction. However, such corrections cannot compensate for the errors introduced by mismatch in the velocity variations of the photoreceptors because these errors differ both in phase and magnitude and are in no way steady or synchronous with the image transfer pitch. For example, a photoreceptor drum characterized by an eccentricity and wobble may rotate with an instantaneous rotational velocity that repeatably varies as a function of the rotational phase angle such that an average rotational velocity over a complete rotation would inaccurately characterize the instantaneous rotational velocity at any single rotational phase angle.
FIG. 2 shows a conventional method for making registration marks and detecting their errors using four chevron marks and two split (bi-cell) detectors. The four chevrons in FIG. 2, denoted KK, MM, CC and YY, are shown in registration to each other. Chevron KK is printed by a black printer, for example printer 1 in FIG. 1. Chevron MM is printed by a magenta printer, for example printer 2 in FIG. 1. Chevron CC is printed by a cyan printer, for example printer 3 in FIG. 1. Chevron YY is printed by a yellow printer, for example printer 4 in FIG. 1. The chevrons are sequentially printed on a process medium, for example intermediate belt 26 in FIG. 1. The process medium carries the chevrons past a pair of fixed detectors D1 and D2 in FIG. 2. Detectors D1 and D2 are typically bi-cell detectors (also called split detectors), although they may be CCD (charge coupled device) detectors. As each chevron moves past a detector, the detector determines the time of passage.
The conventional detection system measures alignment errors in both the process direction 32 and in a lateral direction, transverse the process direction, by determining the alignment error from the times of passage of the centroids of the chevrons past the centers of detectors D1 and D2.
The times of passage are determined as:
T.sub.1KK is the time when the centroid of the KK chevron passes through the center of the split detector D1;
T.sub.1MM is the time when the centroid of the MM chevron passes through the center of the split detector D1;
T.sub.1CC is the time when the centroid of the CC chevron passes through the center of the split detector D1;
T.sub.1YY is the time when the centroid of the YY chevron passes through the center of the split detector D1;
T.sub.2KK is the time when the centroid of the KK chevron passes through the center of the split detector D2;
T.sub.2MM is the time when the centroid of the MM chevron passes through the center of the split detector D2;
T.sub.2CC is the time when the centroid of the CC chevron passes through the center of the split detector D2; and
T.sub.2YY is the time when the centroid of the YY chevron passes through the center of the split detector D2. T.sub.0 is the ideal time at which the centroid of the KK chevron would pass the center of a split detector, V is the velocity of the process medium in the process direction, and S is a pitch distance between the registration chevrons. In order to compute the registration error, perfect knowledge of either S or V is assumed. Assuming that S is known perfectly, the shift in the lateral position of the black chevron from its ideal location is given by: EQU E.sub.KK =0.5*V*(T.sub.2KK -T.sub.1KK).
The shift in the process direction position of the black chevron from its ideal location is given by: EQU F.sub.KK =0.5*V*(T.sub.2KK +T.sub.1KK -2*T.sub.0).
The shift in the lateral position of the magenta chevron from its ideal location is given by: EQU E.sub.MM =0.5*V*(T.sub.2MM -T.sub.1MM).
The shift in the process direction position of the magenta chevron from its ideal location is given by: EQU F.sub.MM =0.5*V*(T.sub.2MM +T.sub.1MM -2*T.sub.0 -2*S/V).
The lateral alignment error of the magenta printer relative to the black printer is given by: EQU E.sub.MM -E.sub.KK =0.5*(T.sub.2MM -T.sub.1MM +T.sub.1KK -T.sub.2KK)*V.
The process direction alignment error of the magenta printer relative to the black printer is given by: EQU F.sub.MM -F.sub.KK =0.5*(T.sub.2MM +T.sub.1MM -T.sub.1KK -T.sub.2KK)*V-S.
In a similar manner, the registration errors of the cyan and yellow printers relative to the black printer are determined.
The computation of the lateral position error involves small differences in time (i.e., T.sub.2KK -T.sub.1KK and T.sub.2MM -T.sub.1MM) multiplied by the velocity. Its accuracy is proportionate to the accuracy with which the velocity is known. The computation of process direction error, however, involves the differences between two large numbers (i.e., 0.5*V*(T.sub.2MM +T.sub.1KK -T.sub.2MM -T.sub.1MM) and S), only one of which has velocity as a factor. Thus, the accuracy of computing the process direction registration error is more highly dependent upon the accuracy of the velocity.
In machine architectures where rotation of the photoreceptor supporting members 1, 2, 3, and 4 and belt drive member 5 (of FIG. 1) are controlled by closed loop servos with feedback from encoders, the run out error of the encoder shaft (eccentricity between the encoder shaft and the roll centers of rotating members 1, 2, 3, 4 and 5) adds to the inherent encoder error and becomes a significant factor.
A major expense in the production of closed loop velocity or position servos is the cost of an encoder. Very accurate encoders (Heidenhain) are priced at a few thousand dollars; encoders of medium accuracy cost a few hundred dollars; and low accuracy encoders cost as little as 25 dollars. To approximate costs, an increase of one order of magnitude in encoder accuracy increases the prices by about one order of magnitude.
Thus, a need exists for a calibration technique that will provide calibration of low accuracy encoders with repeatable rotational phase angle related components of readout errors so that, with only modest increase in cost, they provide the accuracy of the high-priced encoders.
The prior art discloses encoders and methods for improving the accuracy. For example, U.S. Pat. No. 4,593,193 to Michaelis discloses an apparatus and method wherein a servo controller uses generated pulses to calibrate an encoder. A counter is used to keep track of encoder pulses, and a memory stores a calculated error of the pulse. U.S. Pat. No. 4,224,515 to Terrell discloses a high-accuracy optical shaft encoder system wherein an encoder outputs a sine wave. The sine wave is compared to a reference sine wave from a frequency generator and is fed back to control the encoder. U.S. Pat. No. 4,633,224 to Gipp et al. discloses an absolute and incremental optical encoder wherein an algorithm is taught which improves encoder accuracy by using an encoder's absolute position signal and incremental position signal and then comparing these to a stored value. U.S. Pat. No. 3,998,088 to Kazangey discloses a testing apparatus for an incremental shaft encoder wherein a gyroscope is used to accurately test the encoder. U.S. Pat. Nos. 4,792,672 to Spies and 4,806,752 to Fischer each disclose an incremental encoder with a clamping device and a laser rotary encoder.
The concept of using a look-up table for calibration purposes is disclosed in U.S. Pat. No. 4,873,655 to Kondraske wherein a sensor conditioning method and apparatus calibrates a sensor by a look-up table generated by a microprocessor.
The prior art, however, fails to provide a control system using a calibration technique and correction technique for increasing the accuracy of a servo control and removing repeatable rotational phase angle related components of encoder readout errors using a low cost encoder so that it can function with the accuracy of a servo control using a high cost encoder.